1. Field of the Invention
The present invention relates to a semiconductor device having a pixel portion or having this pixel portion and its driving circuit on the same insulator surface. In particular, the invention can be suitably used in a display unit in which a liquid crystal material is provided between electrodes. In this specification, the semiconductor device generally means a device able to function by utilizing semiconductor characteristics, and the above display device, an integrated circuit and an electronic device mounting the integrated circuit and the display unit thereto belong to a category of the semiconductor device.
2. Description of the Related Art
A matrix driving system (this is called an active matrix driving system) for arranging a thin film transistor (hereinafter abbreviated as TFT) every pixel or dot has become an indispensable technique as high definition of pixel density is advanced. The active matrix driving system using the TFT can prevent crosstalk generated in a simple matrix driving system.
Another advantage of the active matrix type display unit is that an integrated circuit such as a shift register, a latch circuit or a buffer circuit can be formed by the TFT on the same insulator as a driving circuit for transmitting a signal to a pixel portion. Thus, the number of contacts with an external circuit can be very reduced and reliability of the display unit can be raised.
Two kinds constructed by a direct viewing type for directly viewing a picture image displayed in the pixel portion and a projection type for projecting this picture image onto a screen by using an optical system are developed in the display unit using a liquid crystal material. In both these types, segregation is considered on the basis of a screen size such that the direct viewing type is used until about 30 inches in screen size and the projection type is used at a screen size of about 30 inches or more.
A liquid crystal is generally operated by an alternating current, and a frame inversion driving system or a line inversion driving system is adopted. In any case, the TFT is used to control a voltage applied to the liquid crystal. Since resistance of the liquid crystal is high, the TFT requires characteristics such as a sufficiently large ON-state current (showing a drain electric current flowing at a turning-on state time of the TFT) able to charge a pixel capacitor (the liquid crystal itself) during a scanning period, a sufficiently small OFF-state current (showing the drain electric current flowing at a turning-off time of the TFT) able to hold electric charges during a field period, a sufficiently small parasitic capacity between gate and drain electrodes, etc. Since pixel capacity is small and a holding operation is insufficient, an auxiliary capacitor is arranged in a pixel to compensate the pixel capacity and prevent an influence of the parasitic capacity.
In the pixel of the active matrix driving system arranging the TFT therein, a scanning line (gate line) connected to the gate electrode and a data line connected to the source or drain electrode cross each other in addition to a pixel electrode for applying a voltage to the liquid crystal. Two kinds constructed by an additional capacity type for overlapping the pixel electrode and a scanning line (gate line) at the previous stage and an accumulating capacity type for arranging a dedicated capacity line are known in the auxiliary capacitor. In any case, sizes of the TFT and the auxiliary capacitor allowed per one pixel are necessarily reduced as high definition of image quality is advanced. Accordingly, it is indispensable to the obtaining of high numerical aperture of each pixel in the prescribed pixel size that elements required in these pixel constructions are efficiently laid out.
A light interrupting film is an element particularly required in the liquid crystal display unit of a transmission type. A semiconductor film has an optical electroconductive effect in which a resistance value is changed by light irradiation. Accordingly, the semiconductor film has an influence on an OFF-state current by irradiating light from a light source to this semiconductor film. In particular, in the display unit of a projection type, a problem exists in that one portion of light emitted from the liquid crystal display unit is reflected on the interface of a substrate and an air layer and is also reflected in an optical system, and is returned in a reverse direction and is incident to the TFT.
In the case of the projection type with a metal halide lamp, etc. as a light source, light of one million to twenty million 1× is irradiated to the liquid crystal display unit so that design of the light interrupting film becomes important. In the display unit of the transmission type, incident light 28 from the light source is incident from the side of an opposite substrate 22 as shown in FIG. 20, and passes through a liquid crystal 27 and is transmitted to the side of an element substrate 21 forming a TFT 23 therein. A light interrupting film 26 is formed on the TFT 23 so that no incident light 28 is directly irradiated to the TFT 23. However, a light component reflected on the interface of the element substrate 21 and the air layer and diffused within the substrate is considered as diffused light 29, and its one portion is incident to a semiconductor film 24. Conductivity of the semiconductor film 24 is raised by the optical electroconductive effect and this raised conductivity has a bad influence on the image display such as an increase in the OFF-state current of the TFT, a reduction in contrast and generation of crosstalk, etc. However, when light interrupting property is preferentially treated and an area of the light interrupting film 26 is increased to interrupt such light, the numerical aperture is naturally reduced.